Pipelines used to transport petroleum fluids such as crude oil and natural gas are commonly wrapped with a jacket of insulating material. For example, in Alaska and other cold climates, insulation is provided along pipelines to prevent the rapid cooling of oil and gas fluids, thus providing better transportability of these fluids. In refineries, pipelines transporting hot fluids are insulated in order to protect personnel from the high temperatures.
In insulated pipelines, the insulation retains moisture around the outside of the pipeline, which moisture promotes corrosion. Therefore, proper maintenance of insulated pipelines requires their periodic inspection for corrosion and other potential leak sources. However, the insulation, which serves as a thermal barrier, also serves as a barrier to inspection with many prior art inspection techniques. Removal of the insulation for inspection and rewrapping of insulation after inspection is both time consuming and expensive. The inspection process is complicated further on those pipelines covered with a metal jacket over the insulation. The metal jacket is used to keep out moisture. The metal jacket is typically provided in two half portions with each portion having flanges for aiding in the retention of the jacket on the pipeline. The two half portions of the jacket are joined together at the flanges which form seams. Water occasionally enters through the jacket seams and travels through the insulation to the pipe where it causes corrosion.
Prior art methods of detecting pipeline corrosion have proven inadequate. For example, pigs with corrosion detection equipment can only be used on pipelines that have access locations providing access to the interior of the pipelines; many pipelines lack such locations. Ultrasonic detection methods require removal of the metal jacket and insulation, a lengthy and expensive procedure. Radiography detection methods are potentially hazardous and the equipment is cumbersome, requiring impractical or inconvenient adjacent vehicular support. Furthermore, with radiography methods it is often difficult to distinguish between corrosion pits filled with corrosion products and uncorroded portions of pipe walls. What is needed then is a method of detecting corrosion through insulation and the surrounding jacket, and which method can be practiced with portable equipment.
Electromagnetic probing techniques provide such a method of detecting corrosion through insulation. In the prior art, frequency domain electromagnetic probing techniques are used to detect corrosion in aircraft fuel tanks. Frequency domain electromagnetic probing techniques utilize a small number of frequencies and measure magnitude and phase differentials between the transmitted signals and the received signals. However, because frequency domain techniques, as a practical matter, utilize only a small number of frequencies, the amount of information obtained is inherently limited, thus detracting from the accuracy of the techniques.
It has been discovered that corrosion of some insulated pipelines occurs in longitudinal bands along the bottom and side portions of the pipelines. Therefore, an antenna configuration that would take advantage of corrosion band geometry would be more effective in detecting corrosion. In the present invention, the receiving antenna is physically separate from the transmitting antenna to achieve better resolution. The transmitting antenna and the receiving antenna are positioned in a loop-loop arrangement or configuration.
The present invention discloses a method for detecting irregularities on the conductive walls of containers, utilizing physically separate transmitting and receiving antennas, wherein the distance between the antenna means and the wall varies across the area of the wall. On insulated containers, the distance between the antennas and the container wall will vary according to the insulation thickness, which thickness is typically nonuniform. Therefore, any method utilizing loop-loop antenna configurations must include steps for correcting for variations in distance between the antennas and the pipe wall.